Are you sure your input is 100% upward and not slightly towards the wall? If it's slightly towards the wall at all, then unfortunately that would be intended behavior, because it matches how it works in melee (a game without many flat wall stages :/ probably why). If you collide with a wall you don't slide along that wall in melee, that's only in Brawl.
On the other hand, if your input is 100% upward or slightly away from the wall, it's a bug. It's a plausible bug, since the game could be checking whether you collided with the wall and deciding yes, but its making an erroneous assessment because you were already touching the wall to begin with.